This invention relates to drill bits of stepped construction for drilling holes of different diameters.
Drill bits of this general category have been known for some time and are disclosed in the following prior U.S. Patents to Tisserant U.S. Pat. No. 1,897,696; Bradley U.S. Pat. No. 3,564,945 and Oakes U.S. Pat. No. 3,758,222. In Tisserant, a stepped drill construction is shown in which adjacent cylindical surfaces are connected by shoulders which form the cutting edges 24. Each cutting edge is disposed at a predetermined pitch for proper cutting clearance. In the Bradley patent each step portion of the drill is "radiused" without any other intermediate surface interconnecting the cylindrical surfaces of the drill. These radiused surfaces provide the cutting edges and each radiused cutting edge is "backed off" and axially "relieved" to provide clearance behind for cutting edges. In effect, the cutting edges thus have a angular pitch, as also disclosed in the Tisserant patent. In the Oakes patent, the drill has a single flute and the cylindrical surfaces are connected by beveled portions which extend helically to provide axial clearance for the drill.